1. Field of the Invention
The present invention relates to a document reading apparatus and a document reading method and more specifically to a document reading apparatus and a document reading method that are applied to digital copying machines or the like which read an image from a document placed on a document reading apparatus or image forming apparatus and form the image on print media.
2. Description of the Related Art
In common image forming apparatus such as digital copying machines, image data read from a document has conventionally been compressed into predetermined coded data before being stored in memory, rather than being stored as is in memory. The image forming apparatus then reads the coded data from memory as requested by the system, decodes it into original image data and outputs the decoded image data to an image forming unit.
For an improved document reading performance, an image forming apparatus with a double-sided reading function (reading both sides of originals as they are transported) is available, as described in Japanese Patent Application Laid-Open No. 10-294830 (1998). Compared with a conventional apparatus using a one-sided reading system, which performs the steps of reading one side of an original to form an image on paper and then flipping the original and reading the other side in an inverted pass to form a back side image on the other side of the paper, the currently available image forming apparatus with a simultaneous double-sided reading function has advantages that because both sides of the original are read at once, the document reading speed does not differ from that of the one-sided reading and that a monotonous document transport path can reduce possible damages to the original.
Further, in the image forming apparatus with the double-sided reading function, it is proposed, as in Japanese Patent Application Laid-Open No. 11-69087 (1999), to arrange image reading sensors in a curved paper path to minimize the size of the apparatus. In reality, to avoid an unwanted backside see-through phenomenon when the document is illuminated, the image reading sensors are disposed at some distance from each other.
In the image forming apparatus with the above-described read-as-document-moves scanning function, however, it is necessary to prepare two image storage devices, giving rise to a problem of an increased cost. This problem in the double-sided reading may be addressed, as described in Japanese Patent Laid-Open No. 8-22536, by temporarily storing the back side image data in a buffer memory to allow the front side image data to be compressed and stored first, followed by the back side image data in the buffer memory being compressed and stored. This enables the image forming apparatus to have only one image storage device.
This arrangement, however, can only apply to a case where the front and back side image data is read almost at the same time or where the completion of the front side reading is followed by the completion of the back side reading.
A user may wish to read images of a variety of sizes of documents, such as A4 and B5 sizes and even smaller sizes of A5 and B6, by the read-as-document-moves scanning method. If a document to be read is as small as A5 or B6, for example, the length of the document in the transport direction may be shorter than the paper pass distance between the front side sensor and the back side sensor. In that case, when a plurality of pages are read, the order of document reading may become irregular, such as page 1 front followed by page 2 front followed by page 1 back. Copying or FAX transmission of such image data as is will pose a problem that the image data will be processed in an irregular order.